In wind turbine systems with variable rotational speed it is known to use a back-to-back converter for transferring electrical energy between the rotor of the double-fed induction generator and the grid. The back-to-back converter comprises a rotor-connected converter, an intermediate DC circuit and a grid-connected converter. The back-to-back converter can be controlled in such a way that the stator windings of the generator can be connected directly to the grid due to the fact that the currents in the rotor windings are controlled in such a way that the frequency of the stator windings corresponds to the grid frequency. Furthermore, the current in the rotor windings is controlled in such a way that desired values of active and reactive power delivered by the generator can be obtained. Such wind turbine systems are e.g. known from U.S. Pat. No. 5,083,039.
In an alternative, the back-to-back converter can be substituted by a so-called matrix converter, said matrix converter corresponding more or less to the back-to-back converter except for not requiring the intermediate DC circuit.
The normal control of such a wind turbine is performed in order to optimize the generated power, said optimization normally being performed by controlling the speed of the wind turbine generator in dependence of the measured wind speed. During such control the electrical power in the generator rotor windings will vary dependent on the rotational speed and other parameters of the wind turbine system. Under certain circumstances relatively high electrical power is present in the rotor windings, and possibly the windings and the electronic components of the converter connected between the rotor windings and the power grid are not dimensioned to the relatively high power to be transferred, and in this connection, possible protective circuits may disconnect the wind turbine from the power grid.